Low-frequency noise characteristics are investigated in top contact type a-Si:H TFT. For analysis of low-frequency noise data, the unified noise model is applied which combine the carrier number and mobility fluctuations. These two mechanisms are confirmed as a main element of noise in a-Si:H TFT by using current-voltage (I-V) measurement and noise analysis. From the measured low-frequency noise characteristics, the effective trap density and scattering coefficient are extracted by using the unified noise model.